I'm thinking of breaking up my contracts into 3 stages. Stage 1: 50% deposit required before any work begins. Stage 2: 25% of contract with client review and approval. Stage 3: Final deliverable w/ client approval and revisions. Let's say I have a client at stage 2 and I send them the invoice for completion and approval.

Do you begin work on Stage 3 once the invoice has been sent, or do you wait to begin work on Stage 3 until Stage 2 has been paid?

I'm considering adding something in my proposals/contracts that states that work for subsequent phases will not be started until previous stage has been completed. Is this normal and/or acceptable?